The measurement of velocities in turbulent and reacting flows is essential for the fundamental understanding and optimization of modern combustors and vehicles. Typical measurement techniques for unsteady flows infer the velocity from pressure, heat transfer or particle movement. They consist of pitot tube pressure measurements, hot wire anemometry, particle imaging velocimetry and laser Doppler velocimetry. These techniques have been applied with great success to a wide array of flows, but have drawbacks since they do not directly measure the molecular motion. Pitot tube and hot wire approaches have limited spatial resolution due to the size of the probes. Particle scattering measurements are not continuous and there are strict requirements for particle seeding: particle size must be carefully controlled to assure that the particles follow the flow accurately and the seeding density must be controlled to provide adequate sampling while avoiding secondary scattering and aliasing.
As an alternative to the techniques mentioned, a number of velocity measurement techniques have recently been applied by directly tracking velocity from molecular motion. These techniques have targeted a number of different molecular species. The first approach for tracking molecules in unseeded air was the RELIEF technique which used two color Raman excitation and laser-induced electronic fluorescence to track molecular oxygen (R. Miles, C. Cohen, P. Howard, S. Huang, E. Markovitz, and G. Russell, “Velocity Measurements by vibrational tagging and fluorescent probing of oxygen,” Opt. Lett. 12, 861-863 (1987); R. B. Miles, J. J. Connors, E. C. Markovitz, P. J. Howard and G. J. Roth, “Instantaneous profiles and turbulence statistics of supersonic free shear layers by Raman excitation plus laser-induced electronic fluorescence (RELIEF),” Exp. in Fluids 8, 17-24 (1989); R. B. Miles, J. Grinstead, R. H. Kohl, and G. Diskin, “The RELIEF flow tagging technique and its application in engine testing facilities and in helium-air mixing studies,” Meas. Sci. Tech. 11, 1272-1281 (2000)). RELIEF takes advantage of the long lifetime of vibrationally excited oxygen. The air photolysis and recombination tracking (APART) technique (N. Dam, R. J. H. Klein-Douwel, N. M. Sijtsema, and J. J. ter Meulen, “Nitric oxide flow tagging in unseeded air,” Opt. Lett. 26, 36-38 (2001)) uses a UV laser to dissociate oxygen and form nitric oxide, which is tracked by laser induced fluorescence. The vibrationally excited NO monitoring (VENOM) technique uses photodissociation of seeded NO2 to produce vibrationally excited NO, which is subsequently imaged by LIF after a delay (R. Sanchez-Gonzalez, R. Srinivasan, R. D. W. Bowersox, and S. W. North, “Simultaneous velocity and temperature measurements in gaseous flow fields using the VENOM technique,” Opt. Lett. 36, 196-198 (2011)). The vibrational excitation allows the tagged NO to be distinguished from background NO. VENOM also has the capability of measuring temperature from the NO fluorescence spectrum. Other molecules which have been tagged for velocimetry include ozone (R. W. Pitz, T. M. Brown, S. P. Nandula, P. A. Skaggs, P. A. DeBarber, M. S. Brown, and J. Segall, “Unseeded velocity measurement by ozone tagging velocimetry,” Opt. Lett. 21, 755-757 (1996); R. W. Pitz, J. A. Wehrmeyer, L. A. Ribarov, D. A. Oguss, F. B. Batliwala, P. A. DeBarber, S. Deusch and P. E. Dimotakis, “Unseeded molecular flow tagging in cold and hot flows using ozone and hydroxyl tagging velocimetry,” Meas. Sci. and Tech. 11, 1259-1271 (2000)), biacetyl (B. Hiller, R. A. Booman, C. Hassa, and R. K. Hanson, “Velocity visualization in gas flows using laser-induced phosphorescence of biacetyl,” Rev. Sci. Inst. 55, 1964-1967 (1984)), and water vapor (L. R. Boedeker, “Velocity measurement by H2O photolysis and laser-induced fluorescence of OH,” Opt. Lett. 14, 473-475 (1989); J. A. Wehrmeyer, L. A. Ribarov, D. A. Oguss and R. W. Pitz, “Flame flow tagging velocimetry with 193-nm H2O photodissocation,” Appl. Opt. 38, 6912-6917 (1999)). Similar approaches have been demonstrated in water (W. R. Lempert, P. Ronney, K. Magee, R. Gee, and R. P. Haughland, “Flow tagging velocimetry in incompressible flow using photo-activated nonintrusive tracking of molecular motion (PHANTOMM),” Exp. in Fluids 18, 249-257 (1995); M. M. Koochesfahani, R. K. Cohn, C. P. Gendrich, and D. G. Nocera, “Molecular tagging diagnostics for the study of kinematics and mixing in liquid phase flows,” in Proceedings of 8th International Symposium on Applications of Laser Techniques to Fluid Mechanics, Vol. 1 (1996) pp. 1.2.1-1.2.12 also in Developments in Laser Techniques and Fluid Mechanics, R. Adrian, et al., eds. (Springer-Verlag, 1997)). Applications of RELIEF enabled the measurement of turbulent fluctuations (A. Noullez, G. Wallace, W. Lempert, R. B. Miles and U. Frisch, “Transverse velocity increments in turbulent flow using the RELIEF technique,” J. Fluid Mech. 339, 287-307 (1997)) and parameters of underexpanded supersonic jets (R. B. Miles, J. Connors, E. Markovitz, P. Howard and G. Roth, “Instantaneous supersonic velocity profiles in an underexpanded jet by oxygen flow tagging,” Phys. Fluids A 1, 389-393 (1989)).
Previous work has used a ultraviolet nanosecond laser to photoionize molecular species and an ion probe to measure velocity (J. M. Ross, G. Laufer, and R. Krauss, “Laser Ion Time of Flight Velocity Measurements Using N2+Tracers” AIAA Journal, Vol 33, #2, February 1995, pp 296-300).